A known linear solenoid linearly drives a movable core through use of a magnetic field that is generated upon energization of a coil of a stator. For example, JP2011-222799A (US2011/0248805A1) discloses a linear solenoid that has a shaft supported by a first stationary core and a second stationary core. The second stationary core includes a bearing portion, a magnetic flux conducting portion (also referred to as an outer tubular portion) and a connecting portion. The bearing portion slidably supports the shaft. The magnetic flux conducting portion is placed on an outer side of the bearing portion in a radial direction and forms an air gap between the magnetic flux conducting portion and the first stationary core in the axial direction. The connecting portion connects between an end part of the magnetic flux conducting portion and an end part of the bearing portion on an axial side, which is axially opposite from the first stationary core.
The movable core includes a holding portion and a magnetic flux conducting portion. The holding portion securely holds the shaft at a corresponding location, which is located between the first stationary core and the second stationary core in the axial direction. The magnetic flux conducting portion of the movable core axially extends from the holding portion at a radial location between the bearing portion of the second stationary core and the magnetic flux conducting portion of the second stationary core. When the coil is energized, the movable core is moved by a magnetic attractive force toward the first stationary core. An axial extent of an overlapped area between the magnetic flux conducting portion of the movable core and the bearing portion of the second stationary core progressively decreases when the movable core is moved toward the first stationary core. In this overlapped area, an axial extent of the magnetic flux conducting portion of the movable core and an axial extent of the bearing portion of the second stationary core are overlapped with each other, and an axial extent of this overlapped area is referred to as the axial extent of the overlapped area.
At the time of energizing the coil, a magnetic attractive force is also exerted from the second stationary core to the movable core to axially attract the movable core toward the second stationary core besides the magnetic attractive force exerted to the movable core to axially attract the movable core toward the first stationary core. The magnetic attractive force, which attracts the movable core toward the second stationary core, is increased when a density of the magnetic flux, which is conducted between the magnetic flux conducting portion of the movable core and the bearing portion of the second stationary core, is increased in response to the decrease in the axial extent of the overlapped area between the magnetic flux conducting portion of the movable core and the bearing portion of the second stationary core. Particularly, the magnetic attractive force, which attracts the movable core toward the second stationary core, is rapidly increased in a latter half of a stroke of the movable core from the second stationary core side (i.e., an initial position) toward the first stationary core side (i.e., a full stroke position). Therefore, the total magnetic attractive force, which is exerted on the movable core, is largely changed in response to the amount of stroke of the movable core.
In order to address the above disadvantage, it is conceivable to lengthen the magnetic flux conducting portion of the movable core and the bearing portion of the second stationary core toward the first stationary core side and to displace the axial position of the first stationary core in such a manner that a sufficient air gap is provided between the first stationary core and the second stationary core. Thereby, the axial extent of the overlapped area between the magnetic flux conducting portion of the movable core and the bearing portion of the second stationary core can be increased. However, this will result in a disadvantageous increase in the size of the linear solenoid.
In JP2011-222799A (US2011/0248805A1), the shaft is configured to reciprocate in the axial direction between the initial position, which is located on the side where the second stationary core is placed, and the full stroke position, which is located on the side where the first stationary core is placed. When the shaft is placed in the initial position, the shaft contacts a yoke made of a metal material. In JP2011-222799A (US2011/0248805A1), the linear solenoid is used as a drive device of a hydraulic pressure change valve of a valve timing control apparatus of an internal combustion engine.
In a state where a magnetic attractive force is not exerted to the movable core, or the magnetic attractive force exerted to the movable core is relatively small, the shaft is driven by an external force or vibration to the initial position to collide against the yoke, thereby resulting in generation of metal collision sound. In the case of the linear solenoid used in the valve timing control apparatus of the engine recited in JP2011-222799A (US2011/0248805A1), at the time of a cranking operation of the engine or at the time of a cleaning operation of the hydraulic pressure change valve of the valve timing control apparatus, when the shaft is moved by the external force or the vibration toward the initial position, the shaft abuts against the yoke to generate the metal collision sound. Particularly, in the case where the cleaning operation of the hydraulic pressure change valve is performed in a state where an engine load is small, a user of the vehicle can clearly hear the above-discussed metal collision sound due to a low level of the engine noise.